Negative Mass in Theoretical Exchange
by meggannn
Summary: Alchemists be thou for the people. And Sherlock is the best alchemist there is, but the hole in Sherlock's chest does not have enough room for the commonwealth, so Sherlock is not for the people. Sherlock is for John. / Fullmetal Alchemist fusion


**Title:** Negative Mass in Theoretical Exchange  
 **Summary:** Alchemists be thou for the people. And Sherlock is the best alchemist there is, but the hole in Sherlock's chest does not have enough room for the commonwealth, so Sherlock is not for the people. Sherlock is for John. / FMA fusion  
 **Genre:** Gen/Supernatural/Magical Realism  
 **Rating:** T  
 **Word Count:** 2k  
 **WARNINGS:** Vague descriptions of blood and organs, implied body horror  
 **Notes:** This is a one-shot I wrote as a side-story to a longer Sherlock/FMA crossover that I never finished back in 2013, a retelling of S1&2 with ~alchemy~. I was cleaning up my GDocs, found this finished, and polished it up as best I could since I'm not sure if I'll ever finish the main fic. I tried to write it so it's not necessary to have knowledge of FMA to read, but as this fic kind of relies on exposition from the main story, here's a rundown: alchemy in this universe is essentially a type of magical science based on the principle of "equivalent exchange," where you can transmute things into other things if a talented alchemist produces an array, or drawn diagram. John is an alkahestrist, a branch of alchemist that specializes in healing, and Sherlock has an extremely rare/nearly unheard of ability to transmute simply by touching his palms together. The main fic, from John's POV, is about him discovering Sherlock's ability; this fic explains how it happened.

* * *

A thousand miles and a death from London, Sherlock takes out an assassin by boiling the water in his blood, all five and a half litres, and leaves him to rot in the Cholistan desert. He walks the two miles back to Bahawalpur, takes a bus to Multan, avoids three bullets, and flies to Jaipur. Five days later, he's in Khon Kaen. Three, Osaka. Eventually there'll be Samara, San Francisco, Edinburgh. He doesn't let himself think about the last flight he'll take, the one to Gatwick a year from now, or two or three or ten, to take out the last piece of the web before a final cab to 221B.

He reminds himself why John couldn't come. John needs chalk, pencils, blood to work. Drawn arrays leave traces. Sherlock only needs his mind.

A sniper in Belize recognizes and greets him with a rigged building set to collapse over his head the moment he steps inside. Sherlock avoids decapitation and gives the man a new face in thanks, and he hopes the message is clear with every bone he breaks across the world, with every destructive clap of his hands that sprays blood and tears muscle: _John Watson is off-limits._

Just one more left, he tells himself with each plane, with each assassin taken out. One more, and then you can go home. It's a lie, of course, a filthy lie, but it helps.

He doesn't think of John when he checks newly disposed targets off his list. He thinks of anything but John.

He thinks of the Truth instead.

* * *

If he had to put it to words now, he'd say the time and space where he'd seen It didn't exist; rationally he knows that cannot be the case, but the whole of it seemed neither here nor there. John would have crowed at Sherlock's inability to put logic to the scene, but he wasn't there to see it, and he isn't here to laugh at him now, and anybody else aware of the matter isn't in a position to share, and so Sherlock will take the scene, and everything it stirs within him, to his grave.

He remembers the whiteness, the emptiness, the hollow space; like a negative mirror of the world's energy. He remembers a door behind him, a tall and sturdy gray door, dark against the white expanse with a thin strip of a shadow underneath where the floor should be. It was, surely, a dream, because no door could stand so rigidly in midair – his head had just reached the bottom edge at the time – and no flat surface could capture his Mind Palace in writing, the intricacies of his mental chambers, the routes and corridors and all the information hidden inside. Yet there it all was, emblazoned in the cleanest black lettering, patterning a stone tablet in the middle of a place that could not exist.

He remembers not feeling… real, quite, every atom of his ten year old body half in and out of reality. As if he'd run a five-kilometer marathon while fasting, trapped immediately before the moment of collapse.

Still a bit dazed from the transmutation's rebound, surely. He'd just have to try the resurrection again later, then; it wasn't as if Carl Powers was going anywhere.

"Hello."

And that was his – his own voice, but he wasn't speaking.

Sherlock turned from the door and started. Sitting in the empty space, staring up at him, was a – a body, a familiar one. He'd nearly missed it, it looked near invisible against the white space. The body of a child, a young boy like himself – Carl. It must be Carl. He'd done it, he'd cracked the secret to bringing the dead back to life. _Mycroft was wrong_ , he could confirm his suspicions with Carl and solve the murder and have his name published in all of the alchemic journals, become the youngest State Alchemist in history at age ten –

The body, the… _thing_ just barely existed, only at its edges: it wasn't not a boy, he revised, but an imitation of one. The bulk was transparent, filled with the space beside it, taking in the nothing to fill the outline; not even a chameleon, something else. Not a living thing at all, really, but – a negative of one. Of Carl? How curious. A hazy outline was the only thing to tell him it existed at all; a shifting, black static outline surrounding the edges of its limbs, like a child had tried to fill in the edges with a scribbling black crayon.

Yet its presence – its resonating, unnerving presence – seemed the most real thing in this empty space; even realer than he felt himself, to be certain, and this wasn't happening, he wanted to say, it's just a hallucination, I must've done it wrong, I'm dreaming, but in the core of it, he knows that it's a lie.

"You're me," he said into the void, and despite himself he sounds younger than he means, like the child he is. It sounds like a confession. He sounds like a sinner.

"Oh, well done." The boy, the other voice – his voice, that's _his voice_ – sounded delighted, preening at its owner, as if it'd been waiting for this, precise moment. "I'm so glad you noticed. Very nice. Now what of your toll? If you haven't decided I'd be happy to pick for you, of course."

"Toll…?"

"Your payment, child."

"You're not – I don't. I don't understand. I already satisfied the equivalent exchange, I gathered all the ingredients to bring Carl back. I – where am I? Who are you really?"

"I am as you understand me, Sherlock Holmes. I am the world. I am the universe. I am God. I am Truth. I am all, I am one, and I am you."

It stood. The same height as Sherlock, meeting his eye at precisely the same height – which makes sense, he thought vaguely, it was his body, or an inverse of his body, the same basic structure even if the colors and physicalities and life needs to be filled in, but that was him and his own voice – and now It was stepping forward. Reaching an arm out, pressing a finger to Sherlock's temple.

"Your brain, I think," It said quietly. It took in no air, exhales no breath upon speaking. "You'd do well to appreciate the irony."

Everything slowed. His thoughts shuttered to a halt, systems failing, activity stalled. He was thinking through molasses, sluggish, and he had a vague sense that he'd lost something, something important, but somehow it didn't seem too bad. He was busy wondering for a name, his name, anything that was still there, but it was fading faster inside, and the edges of his sight were starting to blacken when It removed the finger from his forehead.

The effect was immediate: Sherlock snapped back into awareness, like power had been switched back on, a computer restarting. It all flooded back in, memories and faces and numbers and circles and he grasped for it like a drowning man to a lifeboat –

Hydrogen, 1.00794; Helium, 4.002602; Lithium, 6.941; Beryllium, 9.012182; Boron, 10.811; Carbon, 12.0107; Nitrogen, 14.0067; Oxygen, 15.9994; Fluorine, 18.9984032; Neon, 20.1797, Sodium, Mycroft and Mummy and Carl Powers and the shoes and the human transmutation circle he'd spent so long to design – it was there, god, all there, it was all back again, he'd thought he'd lost it.

God, no, that had been – he'd nearly –

"Ah. No. I see. Your mind won't do. You wouldn't understand." And It reached out another hand, grabbing, expectant, before he could step back with barely a moment to register what was about to happen.

Sherlock felt the peculiar sensation again, more of an ache this time, a bit different than before. A cramp down to his core – something was _wrong_ , something was missing, that thing had done something else, worse – he looked down and gasped for breath, and air flooded into his lungs but the blood wasn't pumping so well, the space felt hollow somehow, oh God.

The hand retreated with its prize.

That was his – bloody and pumping in the center of that thing's chest, but it was his, _that thing's taken_ –

No, god. He just wanted to ask Carl Powers a question. This wasn't what he'd wanted.

"The toll is acceptable." It didn't move as Sherlock collapsed, grabbing at his chest, panting and choking on air (but not out of breath, it's not his lungs It took, God, it's worse, and Mummy, what will she think – )

Sherlock was on his knees, fingers scratching at the skin beneath his shirt – distantly he heard the boom of a door opening, the creak of it widening, and the presence of something More awakening inside, but he paid it no mind. The pain had dulled; his chest didn't hurt anymore, but he'd prefer it too because something was wrong, there was nothing there to be hurt, this isn't what he'd wanted –

"Stand up, child. Your portal has opened. Now I shall show you the Truth."

* * *

Twenty years later, in New York, in Prague, he imagines having told John Watson. He imagines his flatmate, his partner, his best friend faltering when Sherlock confesses everything. "Sherlock, that's." He would pause, take a breath. Try again (doesn't he always?). "You can't survive without your heart. It can't have taken it – not really, not literally. Can it?"

Sherlock would be sitting in his chair next to a dimming fire. Turn a page of his book, _Metallic Alchemical Advancements of the 16 th Century_ (his sixth attempt at trudging through it; he always seemed to lose interest halfway through Chapter 12 at _automail parthenogenic implants_ ), and idly tap a finger on his armchair. To this day the cavity in his chest still offers no answers. If it were anyone else, he would investigate the medial impossibility, picked them apart and preserve the pieces forever. But he's found a limit to his curiosity, the only thing the Truth wouldn't tell him, the only thing he'll ever be satisfied in not knowing.

This, he finds he doesn't want to understand. Whatever has since filled the empty space there (MummyTruthAlchemyKnowledge and now John, _JohnJohnJohn_ ) pumps blood somehow, still keeps him going. It's all just transport anyway. That's all he needs to know.

Be thou for the people. Alchemists be thou for the people.

Sherlock is an alchemist. Sherlock is the best there is. But Sherlock's hole does not have enough room for the rest of the populace, so Sherlock is not for the people. Sherlock is for John.

In Mexico, in Berlin, in Meiringen, in a plane flying to Gatwick Airport, he finally allows himself to think of home.

John had never brought it up. He could be asking that question, _what's replaced your heart, how have you done it?_ back in London now to a gravestone, and Sherlock would never hear, but it wouldn't matter if Sherlock knew or cared. A thousand miles and a death away, he still doesn't respond.


End file.
